The present invention relates to cable continuity testors and more particularly to apparatus for testing continuity and logic faults of multiconductor cables.
As is known in the art, electronic systems such as computer systems typically comprise a number of printed circuit boards electrically interconnected through a "backplane" wiring assembly and cable harness wiring, each of which typically contain a great number of individual wires. To ensure proper operation of the system, the individual wires of the backplane and each cable of the cabling harness are typically tested for continuity after such backplane wiring and cables have been installed in the electronic system, thereby requiring a portable continuity testor.
One method of testing the continuity of the backplane wiring and cables, of course, is to manually test the continuity of each individual wire, for example, with an ohmmeter. However, such manual testing is quite time consuming, labor intensive and error prone. Conventional portable cable testing devices are capable of testing continuity of multiconductor cables either one wire at a time, or in a "daisy-chain" fashion. Another conventional multiconductor cable testor is connected in parallel with the cable under test and checks all of the cable conductors at once by applying a single reference voltage level to the cable conductors at a first end of the cable and comparing the outputs of the cable conductors at the second end of the cable to such reference voltage (i.e., a "static test"). Still another commercially available cable testor is connected in series between the cable under test and the electrical devices to which the cable is coupled during operation. The testor monitors whether signal activity is present on each of the conductors of the cable. While the above described cable continuity testors are portable and thus may be used to test backplanes and cables previously installed in an electronic system, and are thus satisfactory in some applications, they may not be suitable for use in applications requiring testing for binary combinations of signals which will be applied to the cable during operation.
That is, in some applications, especially for backplanes and multiconductor cables used in digital computer systems, it is desirable to test the multiconductor cable or backplane for a multiplicity of logic faults, such as "stuck at zero" (SA0) wires, or "stuck at one" (SA1) conductors, open (i.e., broken or missing) wires, shorted wires, crossed wires, or combinations of such faults. One conventional cable testor capable of testing for the above-identified faults utilizes a microprocessor and memory programmed to control the testing of the backplane or cable, with the apparatus being programmed to perform the various required tests. For example, different logical combinations of signals are applied to the cable under test and compared with the output produced by the cable to determine if any of the above-identified fault conditions exist. While such a microprocessor-based testing apparatus is satisfactory in some applications, it is noted that the apparatus typically is not portable or suitable for use in "field testing" due to the relatively large size thereof and the fact that the apparatus typically must be programmed by trained personnel using specific test programming languages. Also, such a microprocessor-based cable testor is relatively slow, since the apparatus follows a software program for each set of test data applied to the cable, making several decisions based on the resultant output signals from the cable. This can be disadvantageous in applications where the cable or backplane comprises a great number of individual conductors, often requiring several minutes to test a single cable. Moreover, such microprocessor-based cable testors are relatively expensive and require a good deal of support circuitry (e.g., memories, timing circuitry, etc.), and thus utilize a relatively large amount of power.